1. Field of the Invention
This invention relates to a toner used in recording processes such as electrophotography, electrostatic recording, electrostatic printing and so forth.
2. Related Background Art
A number of methods are conventionally known as methods for electrophotography. In common electrophotography, copies or prints are obtained by forming an electrostatic latent image on an electrostatic latent image bearing member (hereinafter also “photosensitive member”) by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner to form a toner image as a visible image, transferring the toner image to a transfer medium such as paper as occasion calls, and then fixing the toner image to the transfer medium by the action of heat and/or pressure.
As methods for rendering the electrostatic latent image visible by the use of a toner, used are cascade development, magnetic-brush development, pressure development, magnetic-brush development making use of a two-component developer composed of a carrier and a toner, non-contact one-component development in which a toner-carrying member is in non-contact with a photosensitive member and the toner is made to fly from the toner-carrying member to the photosensitive member, contact one-component development in which a toner-carrying member is kept in pressure contact with a photosensitive member and the toner is moved from the former to the latter, and jumping development making use of a magnetic toner.
In recent years, as electrophotographic apparatus such as printers, there is a tendency toward higher resolution as a trend of techniques. More specifically, those which hitherto have a resolution of 300 dpi or 600 dpi are being replaced by those having a resolution of 1,200 dpi or 2,400 dpi. Accordingly, developing systems are now required to achieve a higher minuteness. Copying machines have also made progress to have higher functions, and hence they trend toward digital systems. In processes where images are formed by digital processing, chiefly employed is a method in which electrostatic latent images are formed by using a laser. With such digitization, the copying machines are urged to make progress toward higher resolution, and hence they are also required, like the printers, to form images in higher resolution and higher minuteness.
In addition, in the field of electrophotography, color image formation is on rapid progress. Color images are formed by development performed superimposing yellow, magenta, cyan and black four-color toners appropriately, and hence the respective-color toners are sought to have a higher developing performance than those in monochromatic image formation. More specifically, it is sought to provide toners which can develop electrostatic latent images faithfully, can surely be transferred to transfer mediums without scattering and can be fixed with ease to the transfer mediums such as paper.
Accordingly, it has become important to control charge quantity and charge quantity distribution (hereinafter these are termed as charging performance) of toners as uniformly as possible.
The action of charge control agents and the state of adhesion of external additives, described later are chiefly concerned in the charging performance of toners. Then, as techniques for ascertaining the state of adhesion of external additives, techniques are known in which it is defined as the level of liberated external additives (see, e.g., Japanese Patent Applications Laid-open No. H11-258847 and No. 2001-022118).
In these techniques, the level of liberated external additives is controlled by selecting conditions for the step of external addition and the particle diameter and surface state of external additives.
Meanwhile, a publication is also available which argues that both the state of adhesion of a specific external additive and a specific charge control agent used in toner particles are concerned in the image quality (see, e.g., Japanese Patent Application Laid-open No. 2002-055480).
The above publication, however, does not argue about the fact that a specific compound contributes to the achievement of a uniform and reliable state of adhesion of external additives to toner particles to consequently uniform the charging performance of toner and improve the charging performance of toner. Further, it does not argue about the fact that a specific compound shows an auxiliary charge controllability and also it acts mutually with other charge control agent to improve charging performance.
Charge control agents usually used in order to control the charging performance of toners are roughly grouped into two types, a compound having a complex structure wherein a ligand component has coordinated to the central metal and a polymeric compound containing a polar functional group serving as a charging site. The compound having a complex structure has crystallizability and hence has a poor compatibility with binder resins, so that toner production processes may inevitably be limited when it is intended to disperse the compound uniformly in toner particles. In contrast thereto, the polymeric compound type agent has so high compatibility with binder resins as to be readily uniformly dispersed in toner particles, and hence this may place less restriction to production processes and to selection of materials and so forth used in combination.
As the polymeric compound type charge control agent, a resin containing a polymerizable monomer having a specific structure is proposed (see, e.g., Japanese Patent Application Laid-open No. S63-184762).
Meanwhile, toner images formed on the photosensitive member in the step of development are transferred to a recording medium in the step of transfer. Any transfer residual toner at image areas and fogging toner at non-image areas which have been left on the photosensitive member are removed in the step of cleaning, and is stored in a waste toner container. In respect of this cleaning step, blade cleaning, fur brush cleaning, roller cleaning and so forth are conventionally performed. When viewed from the standpoint of apparatus, the apparatus must be made larger in order to provide such a cleaning means. This has been a bottleneck in attempts to make apparatus compact. In addition, from the viewpoint of ecology, a system that may less produce waste toner is desired. Thus, it is sought to provide a toner having a high transfer efficiency and less causing fog.
The charge quantity and charge quantity distribution of toner and the circularity (or sphericity) of toner are concerned in the transfer efficiency.
The transfer efficiency can be high as long as the charge quantity of toner is in a proper range and its distribution is narrow.
If the toner has a low circularity, i) the area of contact of a toner with a drum is large, and hence ii) toner particle surfaces have large unevenness to tend to cause the concentration of electric charges to edge areas, and make large the image force that is produced correspondingly to such areas, resulting in a low releasability of the toner from the drum. That is, in order to improve the transfer efficiency, the toner must be made to have a high circularity.
To make the toner have a high circularity, how to achieve it may differ depending on toner production processes. Processes for producing toners are roughly grouped into a pulverization process and a polymerization process.
The pulverization process is a process in which a binder resin, a colorant and so forth are melt-kneaded to disperse in the binder resin the components other than that, followed by pulverization by means of a fine grinding mill and then classification by means of a classifier to obtain toner with desired particle diameters. In the toner produced by such a pulverization process, the rupture sections caused by pulverization form toner particle surfaces, and hence the toner particle surfaces stand uneven. Hence, the circularity can not be made sufficiently high by such pulverization alone, and it comes necessary to make toner particles spherical by surface modification treatment such as application of mechanical impact or heat treatment as a post-treatment step.
The polymerization process includes production processes of two types, an association agglomeration process and a suspension polymerization process; the former being a process in which, in an aqueous medium containing resin particles formed by emulsion polymerization, serving as a binder resin component, the resin particles and also a colorant, a release agent and so forth are made to undergo association agglomeration in desired particle diameters, and the latter being a process in which a polymerizable monomer composition prepared by dispersing or dissolving a colorant, a release agent, a polymerization initiator and so forth in a polymerizable monomer serving as a binder resin component is made into droplets with desired particle diameters by shear force in an aqueous medium, followed by suspension polymerization.
In the association agglomeration toner as well, its particle surfaces have unevenness caused by the production process. Hence, in order to enhance its circularity, it requires surface modification treatment by a post-step of, e.g., heating the toner particles obtained after agglomeration, or adding a polymerizable monomer composition anew to carry out seed polymerization. The suspension polymerization toner is obtained by polymerizing a polymerizable monomer present in the monomer composition standing in droplets, and hence its particles have a shape closer to a truely spherical shape and have less uneven surfaces, than those in other production processes. Hence, a toner with a high circularity can be obtained without requiring any post-treatment step (see, e.g., Japanese Patent Application Laid-open No. 2001-343788).
That is, the production by suspension polymerization making use of the polymeric compound type charge control agent makes it possible to obtain a toner which is uniformly chargeable and has high transfer efficiency (see, e.g., Japanese Patent Application Laid-open No. 2000-056518).
A technique is also disclosed in which the residual monomer content in toner particles is reduced by using a polymerization initiator having a specific structure (see, e.g., Japanese Patent Application Laid-open No. 2002-251037). There, however, is no disclosure as to the fact that a polymerization initiator having a specific structure forms a specific ether compound and its presence brings an improvement in image quality.
As discussed above, the planning of shape control of toner particles and material design of charge control agents brings an improvement in the charging performance of toners. The controlling of types of external additives and surface treatment thereof and mutual action between external additives and charge control agents also makes it possible to lower the level of liberated external additives and to reduce any contamination due to liberated external additives on members in which the toners participate (in particular, members participating in the development step and the transfer step). However, the both can not simultaneously be satisfied by mere combination alone of these techniques. That is, in the background art, the charging performance required as toners is not sufficiently good, or the member contamination due to external additives is not taken into consideration. Thus, in improving the charging performance synthetically, there has been room for improvement.